Chance Technical Design Manual

ever one is the lesser will control the design. In most cases, the geotechnical resistance (P4) will be the controlling factor. The designer is encouraged to design helical piles so that the geo technical resistance (P4) controls to make the most efficient use of the soil’s ability to bear load. This often means choosing the right shaft type/size, end condition, and helix configuration to maximize capacity. VII. RELIABILITY Reliability is an important aspect of helical pile design. Reli ability is defined as the probability of long-term satisfactory performance. The better the capacity prediction method(s) used, the greater the reliability. Hubbell Power Systems, Inc. recommends using base plus shaft resistance [Method 1] and torque correlation [Method 2] to determine capacity whenever possible. Perko 2009 did a statistical analysis of helical pile capacity in order to check the reliability of this approach. He used a database of several hundred load tests in the analysis and used a factor of safety of 2 to determine a safe allowable load (deterministic approach). Using bearing capacity theory, the load test data suggests that 1 out of 10 helical piles will exhibit unsatisfactory performance. That is a 90% success rate, but still means 10% will have unacceptable performance. Us ing torque correlation, load test data suggests that 0.3 out of 10 will exhibit unsatisfactory performance. That’s a 97% suc cess rate which is much better, but still means that 3% will have unacceptable performance. Methods 1 and 2 are independent methods used to determine helical pile capacity. When two independent methods are statistically combined, the result of poor helical pile performance drops to only 3 piles out of 1000, or 0.3%. That is a 99.7% success rate, which most engineers agree is acceptable reliability. Loads tests [Method 3] is an other independent method of capacity prediction which can be used when soil data is lacking or uncertain, or when soil conditions change. VIII. OTHER TOPICS RELATED TO DESIGN CORROSION POTENTIAL: Underground corrosion is discussed in detail in Appendix A of the TDM. In most ground conditions, corrosion is not a practical concern for deep foundations, in cluding helical piles. There is typically little to no oxygen in un disturbed soils, especially below the ground water table. Driven steel piles have been installed with pile hammers for more than a century and are still commonly used today. The vast major ity of interstate highway bridges in the Piedmont regions of the southeast United States are bearing on driven steel H-piles. If the geotechnical report declares the corrosion potential is moderate to severe for a given project, then a square shaft heli cal pile is a good choice because of its solid cross section and low perimeter surface area compared to a pipe shaft; which is hollow and has more perimeter surface area relative to the cross-sectional area of steel. Hot-dip galvanization adds a thick coating of zinc to the steel pile. It provides a durable coating that increases service life. Service life calculations based on metal loss rates can be done when corrosion potential data is

The allowable helix strength (P3) must equal or exceed the end-bearing capacity (P4) of the of the helix plates. It is pos sible for the bearing capacity of a helix plate to exceed the structural strength of the helix plate For example, an SS175 10” diameter helix plate has an allowable strength of 33.1 kip per Table C-10. If the maximum allowable torque based capacity of an SS175 helical pile (52.5 kip) is needed, then more than one 10” helix is required to meet structural strength requirements since 33.1 kip is less than 52.5 kip. A twin-helix or triple-helix configuration will work. This is an example where the designer may want to specify a minimum number of helix plates in the project plans. As helix plate diameter increases, the helix strength (P3) gener ally decreases. This is because the line of bearing (average ra dius) increases with increasing diameter, which in turn increas es the moment arm distance. The increased distance increases the bending forces at the helix/shaft welded connection. Load tests [Method 3] are used to verify the feasibility and ca pacity of helical piles/anchors and are described in detail in Appendix B of the TDM. They can be part of a pre-production test program where at least one helical pile is installed and tested to determine the ultimate resistance and the load/de flection response. Project requirements may also require pro duction tests on a specified number of helical piles/anchors to ensure capacity and performance requirements are being met. It is VERY IMPORTANT that the performance requirements be clearly specified BEFORE the start of work. It should be part of the data gathering process and feasibility assessment for helical piles. Helical piles are primarily end-bearing founda tion elements, meaning they derive most of their resistance with the helix plates transferring load to the soil at the pile tip. Therefore, the load/deflection response of a helical pile at a particular load (serviceability) must take into account the sec tion modulus and length of the shaft. The designer must under stand that long end-bearing piles will displace more than short end-bearing piles because of the pile length. The recommended acceptance criteria for the allowable ca pacity of helical piles/anchors is 50% of the applied test load causing a net displacement equal to 10% of the average helix diameter (D ave ). This means that total displacement of the pile/ anchor may exceed 1 inch in order to fully mobilize the bearing capacity of the helix plates. This is the acceptance criteria used in ICC-ES Acceptance Criteria AC358 for Helical Systems and Devices, per Section 4.4.1.2. It can be expressed mathemati cally as PL/AE + 0.10D ave , where “PL/AE” is the elastic shorten ing or lengthening of the pile shaft under load. As mentioned previously, the net displacement of the helix plates at allowable loads will average 0.25 in (6.4mm) ± 0.12 in when using a geo technical factor of safety of two. VI. SUMMARY In summary, helical pile design determines the geotechnical re sistance (P4) and structural capacity (P1, P2, & P3), typically in that order. Probe helical piles and load tests are often done before start of work when a geotechnical report is not available or when verification of capacity is required. The geotechnical and structural resistance are separate limit states and which

HELICAL PILES & ANCHORS

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